In a hybrid electric vehicle or HEV, an electric machine or motor/generator can be used in conjunction with a high-voltage battery module in order to selectively propel the vehicle. In other vehicle designs, such as in a purely electric vehicle or PEV, the electric machine and battery module are used as the exclusive power source for vehicle propulsion. An electric machine configured as an induction-type AC device of the type known in the art can produce rotational motion or motor torque in response to an applied electrical current, and when operating in a generator mode the torque provided by the same device can be directed to generate electricity suitable for recharging the battery module, and/or for other useful purposes.
In particular, a conventional electric machine includes a rotatable member or rotor, a stationary member or stator, and a rotatable output shaft connected to or formed integrally with the rotor. An air gap can separate the stator and rotor. The main magnetic flux passes through the rotor core, as well as a rotor excitation layer, e.g., a permanent magnet, the air gap, and the stator core to thereby form a closed path, with the closed path magnetically linking the stator windings of the electric machine. The induced voltage within the electric machine is proportional to the total flux linking the stator windings.
To ensure proper operation of the motor/generators, as well as to optimize fuel economy of the vehicle, a motor controller or electronic control unit (ECU) can perform various measurements and/or diagnostics of the various electrical and mechanical systems aboard the vehicle, and can control the various modes of the electric machine as determined by programmed motor control logic. A high torque/ampere ratio and wide speed range are essential in certain applications, such as automotive propulsion and power generation applications. Since induced voltage is proportional to motor speed, a higher induced voltage can limit the available speed range of the electric machine as it approaches system bus voltage at a lower speed.
Typical machine control techniques account for this condition by selectively weakening the magnetic flux in the electric machine. For example, the amperage delivered to the stator windings can be increased in order to weaken the induced magnetic field. Such a technique can result in less than optimal machine efficiency while increasing inverter current requirements. Additionally, the power factor of the machine can be reduced using conventional methods, thus requiring more electrical current for a given required power output of the electric machine.